What is viscosity index?

It is the rate of change of viscosity of an oil with respect to change in temperature.

An oil with low viscosity index has greater change of viscosity with change in temperature.

An oil with high viscosity index has very little change of viscosity with change in temperature, which is a desirable property for lubricating oil.

For crankcase oil, viscosity index is 75 to 85.

For cylinder oil, viscosity index is 85.

Viscosity index of mineral oils is highest, about 115 and this may be raised to about 160 by adding special additives.

Hydraulic oils should have high viscosity index for faster response of the system. It is usually around 110.

What is a pour point?

It is the lowest temperature below which an oil will stop flow.

Pour point indicates that oil is suitable for cold weather or not.

Pour point of engine crankcase should be -18°C.

What is the flash point?

It is the lowest temperature at which the oil will give off a sufficient inflammable vapour to produce a momentary flash when a small flame is brought into the surface of the oil.

Close flash point for crankcase lubricating oil is around 220°C.

Why flash point is important?

Fuel oil flash point is to be high because if it is low, there would be a possibility of fire in storage.

Engine crankcase lubricating oil flash point should be as high as possible to prevent crankcase explosion.

For safe storage, oil storage tank heating temperature to be limited to at least 14°C lower than flash point to prevent fire.

Average Closed Flash Points

Petrol = -20°C

70cSt Fuel Oil = 71°C.

Paraffin = 40°C

Lube Oil = 220°C

Diesel Oil = 65°C

What is Total Acid Number or TAN and Total Base Number or TBN?

The ability of an oil to react with a base reagent which indicates the acidity is expressed as TAN.

The ability of an oil to react with acidic reagent which indicate the alkalinity is expressed as TBN.

The results are expressed in milligrams of potassium hydroxide (KOH) required to neutralise one gram of sample oil for both TAN and TBN.

TBN for an oil used for cross head type diesel engine crankcase is 8mg KOH/gram of oil.

TBN for an oil used for trunk type engine using heavy oil is 30mg KOH/gram of oil.

What type of engine are using high TBN and why?

If blow pass occur in a trunk type engine using heavy fuel oil, incomplete combustion products reach directly into the crankcase and may cause the contamination of lube oil with acid. Thus in this type of engine to neutralize the acid contamination must be used high TBN oil.

What is detergency and dispersancy ?

It is a chemical additive called detergent which has a property of preventing the deposition of carbon deposits and wash away with the lube oil.

Dispersant additive is added to divide the larger size deposits into tiny particles to be carried in a colloidal suspension evenly throughout the bulk of oil, which can be removed while filtration of the oil.

Explain the L O sampling procedure to send for LO test ?

The sample should be drawn with oil circulation with the system such as a test cock on the discharge side of the LO oil pump.

Before taking the sample oil sufficient amount of oil should be drain out to clear the line.

The sample is filled into the chemically cleaned container after it is rinsed with the sample oil and immediately closed.

The container should be attached with information on engine type, engine running hours, LO running hours, fuel used, draw off point and date of sampling.

Explain Sampling Procedure of lube oil?

Draw samples from a connection that comes directly out of the main oil supply line to the engine.

Always sample for the same point.

Sample only when the oil is up to its operating temperature with the engine running.

Depending upon the draw off point, sufficient amount of oil should be drained out of the line prior to drawing the sample.

The sample should be filled into a chemically cleaned container after it is rinsed with sample oil and immediately closed.

The container should be attached with a label as follows:

Records for Sample

Date of sample drawn

Point of sample drawn

Type of oil

Type of machinery use

Temperature of sample drawn

The period of time since the last renewal of oils.

Avoid sampling from places where the oil may be stagnant or have little or no flow, such as sumps, auxiliary smaller pipelines, purifier suction or discharge lines, drain cocks of filters, coolers etc.

Also avoid sampling while engine is stopped.

What is the essential factor effecting the establishment of hydrodynamic lubrication ?

Viscosity of lube oil

Load acting on the bearing

Surface smoothness of moving parts

Speed of rotation

Continuous LO supply

Bearing clearance, bearing length and pin diameter.

There must be convergence between fixed end and moving surface.

Where do locate ME LO sump and its fitting ?

It is located under the main engine in the ship double bottom and surrounded by cofferdams.

What is meant by boundary lubrication?

High friction between the surfaces and some degree of metal to metal contact occurs

Lubricant oil film decreases, until asperities of mating surfaces touch

What is meant by hydrostatic lubrication?

A form of thick film lubrication, but instead of being self- generated, it is supplied from an external source by oil under pressure from a pump.

Lubrication for Crosshead Bearings, with attached pump.

What is meant by elasto-hydrodynamic lubrication?

Applies to line contact or nominal point between rolling or sliding surfaces, such as rolling contact bearings and meshing gear teeth.

Thin film or squeeze film lubrication limits metal to metal contact.

Elastic deformation of the metals occurs, and there is effect of high pressure on the lubricant.

What will you do as 2/E, if lube oil is contaminated with sea water?

When sump oil is contaminated with SW, find sources of leakage (may be from LO cooler during ME stopped) stoppage and rectified.

In port or while ME is stopped, transfer contaminated oil through purifier or transfer pump intosettling tank, settled for at least 24 hours at about 60 C°, and water and sludge drained out periodically.

Oil passed through purifier at 78° C with optimum efficiency, and pump back to settling tank.

When sump tank is empty, interior cleaned and examined.

Purified oil sent to laboratory and tested

During this time, new oil should be used

Oil should be reused, if lab results recommended that it is fit for further use. (Straight mineral oil 3% water washed. Additive oil 1% water washed).

What are the difference in Cylinder oil and System oil?

Cylinder oil is detergent / dispersant oil

System oil is straight mineral oil

What are the lube oil tests carried out on board Ships ?

Viscosity Determination

The simplest method is three tube rolling ball viscometer

Assuming the oil in the engine to be SAE 30 grade, one tube is filled with minimum safety viscosity (about SAE 20) and another one filled with maximum safety viscosity (about SAE 40). The last tube is to be filled with test sample.

All tubes are placed in a bucket of warm water until the oils are at the same temperature.

The three tubes then mounted on a tilted board and inverted. An internal hollow ball in each tube then rises to the surface.

If the time taken in the test sample is between that of the lower and upper limit oils, the oil is fit for further use. If not, it must be replaced.

Insoluble Content

A drop of sample oil is released from a given height onto a special filter paper.

Compare the result with the known varying insoluble content. The upper limit for straight mineral oil is 1% to 1.5 % and for detergent dispersant oil is 5%.

Water and other Contaminants

A known amount of sample oil in the test tube is heated and must be shaken the while doing so

If there is no cracking, the oil is dry

If there is slightly cracking, the oil having a trace of water

If there is a heavily crackling, the oil is heavily diluted with water

Acidity / Alkalinity Determination

Acidity is tested by extracting the acids from the sample by means of shaking with a known amount of distilled water. The acidic extract is then placed on a watch glass with an indicator solution of known strength. The mixture is then drawn up into a glass tube and its colour compared with a series of colour standards, each representing a known PH value, from which the sample can be determined quite accurately.

Another method is :

A drop of indicator solution is placed on to blotting paper and this is followed by a drop of sample oil placed at the centre of the drop of previous absorbed indicator.

If the change of colour is Red, it is acid

If blue/green, it is alkaline.

If yellow/green, it is neutral.

Foreign Particles Test

This can be done by either Spectrochemical analysis or Ferrographic analysis, each giving particle size less than 10 uM to 100 uM range depending on the tests applied.

The most powerful technique is Inductively coupled plasma atomic emission spectrometry (ICP OR PES), which uses a direct spray technique to determine the wear and contaminant elements present in the oil. This technique will in the main only detect the particles below 10 uM size.

In ferrographic test, the sample is thinned first with some solvents and allowed to pass slowly down a slide surrounded by powerful magnetic field. Then it is examined by special microscope with red and green filters under lights. The shape of the particles is used to identify the source of the wear debris.

The advance ferrography method was added to the PFA (Progressive Fast Analysis) programme where all samples pass through the combination of two machines, a particle quantifier (PQ) and a rotary particle depositor (RPD). These test measures the induced magnetic moment of debris as deposited on a substrate or contained within a specific volume of liquid.

These machines provide accurate test by rotating the metal particles and then separating into three different sizes, Theses three bands of particles are examined by very powerful microscope to determine the type of materials and shape.

If more detailed examination is necessary, the debris may be subjected to a scanning electron microscope.